Current limiter utilizing a small ohmic resistance in series with the load



Feb. 25, 1964 P. MUCHNICK ETAL CURRENT LIMITER UTILIZING A SMALL OHMIC RESISTANCE IN SERIES WITH THE LOAD Filed May 27, 1960 l2 l4 l5 lo A. C. Sotumble Rectifier Finer Supply Reactor D. C. 42' Amplifier Voltage Reference PAUL MUOHNICK ERNEST LEVY, JR.

INVENTORS ATTORNEY United States Patent ice 3,122,694 CURRENT LIMITER UTILIZING A SMALL OHMIC RESISTANCE IN SERIES WITH THE LOAD Paul Muchnick and Ernest Levy, Jr., Norwalk, Conn.,

assignors, by mesne assignments, to Raytheon Company, a corporation of Delaware Filed May 27,. 1960, Ser. No. 32,359 7 Claims. (Cl. 321-16) countered during the operation of the regulator circuit.

.fWhen exceptionally high current pulses do occur it is )necessary to provide additional protection to prevent damage of components. The present invention is designed with this end in view and includes three transistor amplifier units which are normally non-conducting and only conduct current when the current through a resistor is increased to a predetermined maximum value. Such a condition may result from the short-circuiting of the load terminals.

One of the objects of this invention is to provide an improved current limiter which avoids one or more of the disadvantages and limitations of fuses and other prior art current limiting devices.

Another object of the invention is to limit the current in voltage regulator circuits under abnormal conditions such as short-circuiting of the load terminals.

Another object of the invention is to provide a current limiter which is exceptionally fast in action and does not have the usual time interval or delayed action which is characteristic of fuses.

Another object of the invention is to provide a current limiter which will be restored to its normal condition when the over current conditions are rectified.

Another object of the invention is to provide a current limiting circuit which may be adjusted to act at any desired value of current.

The invention comprises a small resistance, of the order of one or one-tenth ohm, connected in series with a direct current load. The input terminals of an amplifier are connected across the ends of this resistor in series with an adjustable bias voltage. The output terminals of the amplifier are connected across a control winding mounted on the core of a saturable reactor. The amplifier is adjusted to pass no current under normal operating conditions but as soon as the voltage across the series resistor overrides the bias voltage in series with the amplifier input terminals, a large current is sent through the control coil and the current is reduced to a normal value.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawing.

The figure is a schematic diagram of connections showing the current limiter in detail but with some of the other circuits shown in block.

Referring now to the figure, the circuit includes a pair of input terminals 10, 11, which are to be connected to a source of alternating current power which may vary in applied voltage. The terminals are connected to a saturable reactor 12 which may be any one of a number of saturable core types which are well-known in the art and which have been described in prior publications.

3,122,694 Patented Feb. 25, 1964 The reactance of reactor 12 is controlled by a coil 13 which is mounted on the reactor coil and controls the saturation of the core by a variation of direct current through the coil winding. The saturable reactor is coupled to a rectifier 14 which contains rectifier units and produces pulsating current which is applied to a filter circuit 15. The output of the filter circuit is connected to output terminals 16 and 17 for connection to a direct current load 18. In one of the conductors between the filter circuit 15 and one of the output terminals a small resistor 20 is connected. This resistor may be of the order of one-tenth to one ohm and is used specifically for the detection of the current flowing through the load 18.

It is contemplated that the current limiter circuit will be used in conjunction with a voltage regulator circuit and since this is the preferred embodiment, the circuit will be described with this combination. It should be pointed out, however, that the current limiter circuit as described can be used without the voltage regulator circuit. The voltage regulator circuit includes a voltage reference circuit 21 which is connected across the output terminals 16, 17, in series with an adjustable resistor 22. Any type of voltage reference circuit may be used and the adjustable resistor 22 may be omitted. The voltage reference circuit 21 delivers an error voltage, above or below a predetermined desired voltage value. The error signal is applied to a direct current amplifier circuit 23 and the output of the amplifier is applied directly to the control coil 13 on the saturable reactor core. The voltage reference circuit 21 and its associated direct current amplifier 23 are old in the art and need not be described in de-,

tail here.

The current limiter circuit includes an amplifier 24, a biasing circuit 25, and a potential supply 26. The amplifier 24 in this embodiment includes three transistors 27, 28, and 30 coupled to each other in the usual manner to amplify direct currents.

The amplifier 24 includes input terminals 31A and 32 and output terminals 31B and 33, the terminals 31 being the common conductor for both input and output circuits. The input terminals of the amplifier are applied across the ends of resistor 20 in series with a biasing potential which is a portion of a voltage divider 34. The voltage divider 34 is connected across a zener diode 35 which derives its applied voltage from a transformer 36 connected through a rectifier diode and a series dropping resistor 38. The primary winding 40 of transformer 36 is connected directly across input terminals 10 and 11 and the secondary winding 41 is connected to the zener diode 35 in series with rectifier 37 and resistor 38. The voltage divider 34 is adjusted so that all three transistors are far below their cut-oft potential and considerable input voltage is required to overide this bias and make the amplifier 24 conduct.

The output terminals 31B, 33, of amplifier 24 are connected in series with winding 13 on the saturable reactor and the source of potential 26 which includes a transformer 42 having a primary winding 43 connected directly to the input supply terminals 10* and 11 and a secondary winding 44 connected to one side of winding 13 and to output terminal 33 in series with a rectifier component 45.

The operation of this circuit is as follows: Under normal conditions the alternating current supply is coupled through the saturable reactor -:12, rectifier 1'4, and filter 15 to supply direct current to load 18. The voltage across the output terminals is sensed by the voltage reference circuit 21 and the error voltage, if any, is amplified by amplifier 23 and applied to the control winding 13 on the saturable reactor core to regulate the voltage across terminals "16 and 17 and maintain it within a narrow restrictive range of values. This is the normal operation of the circuit and is old in the art.

amass s If a short circuit should be applied to terminals 16 and 17, an abnormally large current will flow through the supply components, the voltage across the output terminals is reduced to Zero, and the voltage reference circuit will generate a large negative error voltage which generally is too large "for the direct current amplifier to handle. The large currents produced by the short circuit may darnage many of the components in the circuit and it has been found that a fuse or circuit breaker is too slow acting to give adequate protection. The current limiter circuit protects the circuit components by producing almost immediate protective action. The large current through resistor 20 produces a voltage which overides the bias voltage V furnished by the voltage divider 34 and all of the transistors in )amplifier 24 conduct and a large current is sent from the output terminals 3 1B, 33, through control coil '13, overriding the current produced by amplifier 23, to increase the reactance of reactor 12 so that the current through the rectifier and filter units is reduced to a low value and all circuit components are protected. The current limiter circuit as shown maybe used in combination with circuit breakers or ifuses which will permanently dis connect the load until connections are manually restored. As shown in the figure, the circuit provides protection against short circuit and as soon as the short circuit is removed the device continues its normal operation without supervision.

In the foregoing description, amplifier 524 has been shown as a three stage transistor amplifier because this is the preferred embodiment and because it produces sufiicient currents to afford the protection desired. However, other amplifiers can be used, including vacuum tube amplifiers, as long as suflicient output current is available and as long as the amplifier handles direct current.

The foregoing disclosure and drawing are merely illustrative of the principles of this invention and are not to be interpre ed in a limiting sense. The only limitations are to be determined from the scope of the appended claims.

We claim:

1. A current limiter circuit for limiting the current supplied to a load comprising in combination, a pair of input terminals for the application of alternating current power, a saturable reactor coupled to said input terminals and including a control Winding fior controlling the magnetic saturation of a core and the impedance of the reactor, a pair of output terminals for connection to a direct current load, a rectifier and filter coupled to said saturable reactor for converting alternating current into direct current and applying the direct current to said output terminals, a resistor connected in series with one of said output terminals for generating a voltage wihch is proportional to the current supplied to the load, and a direct current amplifier having input terminals connected across said resistor and in series with a bias voltage, said direct current amplifier having output terminals connected across the control winding in series with a supply voltage source, said amplifier adjusted to pass current only when the voltage across said resistor is raised above a predetermined value.

2. A current limiter as set forth in claim 1 wherein a voltage reference circuit is connected across the load for the generation of an error signal which is applied to said control winding'in series with a second amplifier for maintaining the load voltage within a restricted range of values.

3. A current limiter :as set forth in claim 1 wherein the bias voltage is derived from said alternating current input terminals coupled through a rectifier and wherein said bias voltage is stabilized by means of a zener diode.

4. A current limiter as set forth in claim 3 wherein the bias voltage is made manually adjustable by means of an adjustable voltage divider connected across said zener diode.

5. A current limiter as set forth in claim 3 wherein said supply voltage source in series with the control winding is derived from the alternating current terminals coupled through a rectifier.

6. In a current limiter for limiting the current supplied to a load normally controlled by a voltage regulator, the combination comprising, a pair of input terminals for the application of alternating current power, a saturalble reactor coupled to said input terminals and including a control winding for controlling the magnetic saturation of a core and the impedance of the reactor, a pair of output terminals for connection to a direct current load, a rectifier and filter coupled to said saturable reactor for converting the alternating current into direct current and applying the direct current to said output terminals, a voltage reference circuit connected across the output terminals for the generation of an error signal above or below a predetermined voltage value, a first direct current amplifier coupled between said voltage reference circuit and said control winding for increasing the reactor impedance when the 1' error signal is positive and for lowering the impedance when the error signal is negative, a resistor connected in series with one of said output terminals for generating a voltage across its terminals which is proportional to the current supplied to the load, and a second direct current amplifier having input terminals connected across said resistor and in series with :a bias voltage, said second direct current amplifier having output terminals connected across the control winding in series with a supply voltage source, said amplifier adjusted to pass currently only when the voltage across said resistor is raised above a predetermined value. a

7. In a voltage regulator having a pair of input terminals tor the application of alternating current power, a saturable reactor coupled to the input terminals and including a control winding for controlling the magnetic saturation of a core and the impedance of the reactor, a pair of output terminals for connection to a direct current load, a rectifier and 1a filter coupled to the saturable eactor for converting the alternating current to direct current and applying the direct current to the output terminals, a Voltage reference circuit connected across the output terminals for generating an error signal above or below a predetermined voltage value, a first direct current amplifier coupled between the voltage reference circuit and a control winding lfor varying the reactor imed'ance in accordance with the error signal voltage, the combination comprising: a resistance connected in series with one of the output terminals for developing a voltage thereacross in proportion to the current supplied to the References Cited in the file of this patent UNITED STATES PATENTS 2,875,395 Blashfield Feb. 24, 1959 2,888,633 Carter May 26, 1959 2,914,720 Merkel Nov. 24, 1959 3,042,848 Muchnick et al July 3, 1962 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3, 122,694 February 25 1964 Paul Muchnick et ale It is hereby certified that error afapears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 34, for "currently" read current Signed and sealed this 30th day of June 1964.

(SEAL) Attest:

ERNEST W; SWIDER EDWARD J. BRENNER Aziesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,122,694 February 25, 1964 Paul 'Muchnick et a1,

It is hereby certified that error aibpear s in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 34, for "currently" read current Signed and sealed this 30th day of June 1964,

(SEAL) Attest:

ERNEST W; SWIDER EDWARD J. BRENNER Afsesting Officer Commissioner of Patents 

7. IN A VOLTAGE REGULATOR HAVING A PAIR OF INPUT TERMINALS FOR THE APPLICATION OF ALTERNATING CURRENT POWER, A SATURABLE REACTOR COUPLED TO THE INPUT TERMINALS AND INCLUDING A CONTROL WINDING FOR CONTROLLING THE MAGNETIC SATURATION OF A CORE AND THE IMPEDANCE OF THE REACTOR, A PAIR OF OUTPUT TERMINALS FOR CONNECTION TO A DIRECT CURRENT LOAD, A RECTIFIER AND A FILTER COUPLED TO THE SATURABLE REACTOR FOR CONVERTING THE ALTERNATING CURRENT TO DIRECT CURRENT AND APPLYING THE DIRECT CURRENT TO THE OUTPUT TERMINALS, A VOLTAGE REFERENCE CIRCUIT CONNECTED ACROSS THE OUTPUT TERMINALS FOR GENERATING AN ERROR SIGNAL ABOVE OR BELOW A PREDETERMINED VOLTAGE VALUE, A FIRST DIRECT CURRENT AMPLIFIER COUPLED BETWEEN THE VOLTAGE REFERENCE CIRCUIT AND A CONTROL WINDING FOR VARYING THE REACTOR IMPEDANCE IN ACCORDANCE WITH THE ERROR SIGNAL VOLTAGE, THE COMBINATION COMPRISING: A RESISTANCE CONNECTED IN SERIES WITH ONE OF THE OUTPUT TERMINALS FOR DEVELOPING A VOLTAGE THEREACROSS IN PROPORTION TO THE CURRENT SUPPLIED TO THE LOAD, A SECOND DIRECT CURRENT AMPLIFIER HAVING INPUT TERMINALS CONNECTED ACROSS THE RESISTOR AND IN SERIES WITH A BIAS VOLTAGE, THE SECOND DIRECT CURRENT AMPLIFIER HAVING OUTPUT TERMINALS CONNECTED ACROSS THE CONTROL WINDING AND IN SERIES WITH A SUPPLY VOLTAGE SOURCE, THE SECOND AMPLIFIER BEING ADJUSTED TO PASS CURRENT ONLY WHEN THE VOLTAGE ACROSS THE RESISTOR IS RAISED ABOVE A PREDETERMINED VALUE. 